1. Field of the Invention
The present invention relates generally to a periodic signal detector for use in controlling the operation of an echo canceller, a noise canceller, and the like.
2. Prior Art
Heretofore, an echo canceller has been known which estimates an echo flowing through a channel to subtract the echo from an echo signal transmitted to the channel, thereby cancelling the echo. The echo estimation is done by updating coefficients of an adaptive filter for generation of an echo replica signal.
In a case where, however, such an echo canceller receives a signal having high periodicity (periodic signal) such as a sine-wave signal, the updating of the filter coefficients is degraded. It is therefore preferred that the echo canceller suspends the updating of the filter coefficients when it receives the periodic signal. For example, signals having high periodicity for a telephone system include a tone signal, a push-button dial (DTMF: dual tone multi-frequency) signal, a holding tone and the like.
The suspension of the updating of the filter coefficients in response to an input of the signal having high periodicity requires the detection of the signal having high periodicity. A technique for detecting the signal having high periodicity includes a method which uses FFT analysis to detect the signal having high periodicity. This method, however, requires large amounts of data to be processed and much time for detection, and therefore is difficult to apply to the echo canceller.
In view of the foregoing, it is an object of the invention to provide a periodic signal detector adapted for quick detection of a signal having high periodicity by using small amounts of data to be processed.
A periodic signal detector according to the invention comprises: sample counting means for counting a number of samples between adjacent zero cross points in an input signal, the sample counting means outputting the number of samples each time the number of samples between the adjacent zero cross points is obtained and continuing outputting the number of samples until a next zero cross point is detected; and detecting means for detecting a section, as a periodic signal section, in which the output from the sample counting means has a constant result.
The detecting means used herein comprises, for example, a high-pass filter receiving the output from the sample counting means for outputting, as a zero output value, the section in which the output from the sample counting means has the constant result, zero counting means for counting the number of successively sampled zeros included in the output from the high-pass filter, and judgment circuit for detecting a section, as the periodic signal section, in which an output from the zero counting means is not less than a predetermined threshold value.
Preferably, the periodic signal detector further comprises a low-pass filter for suppressing an unwanted zero cross variation due to a noise in the input signal, an output of which low-pass filter is supplied to an output to the sample counting means.